1. Field of the Invention
The present invention relates to a rear-projection display, and more particularly, to a rear-projection display having an optical system which includes an auxiliary mirror placed between a light source and a liquid crystal display (LCD) panel which transmits a portion of incident light and reflects another portion thereof to provide uniform brightness of images projected on a screen.
2. Description of the Related Art
Recently, the demand for large image displays has increased. However, there are practical limitations to achieving large image displays because the size of a cathode ray tube (CRT) must be large in order to obtain a large screen. Even if the technical problems of manufacturing such a large CRT can be resolved, the increased weight of the CRT would impose constraints on manufacturing a television set or the like employing the large CRT as a display. Thus, alternative ways to obtain a practical large display screen have been explored. Image projection systems such as projection televisions or video projectors are examples of such alternatives.
In the above-noted image projection systems, an image is generated by using an image displaying device such as a small CRT or an LCD, then magnifying and projecting the image with an optical system onto a large screen. The desire for a large screen has made systems like this commercially popular.
Image projection systems can be grouped into front-projection types and rear-projection types, depending on the mechanism used to magnify and project an image onto the screen. A typical front-projection display is provided with a white light lamp as a light source and three transmission LCDs. White light emitted from the white light lamp is separated into three colors (red, green and blue). Each separated color is illuminated on an LCD which displays an image corresponding to a video signal for that color. The colored lights which have passed through their respective LCDs are combined by a color discriminating mirror, are superposed, and are directed to be incident on a projection lens which projects the image displayed on each LCD onto a screen as a single color image.
However, such a front-projection display has a distinct drawback in that, due to its structure, there is poor contrast on the screen when the viewing surroundings are bright. Therefore, the surrounding illumination must be as dim as possible, like that of a movie theater, in order to improve the contrast on the screen. In addition, as a viewer deviates from a position which is directly perpendicular to the plane of the screen, the contrast of the displayed image decays.
In an effort to overcome the drawbacks of the front-projection display, rear-projection display (FIG. 1) devices have been developed. The rear-projection display can be made slimmer than a front-projection display, and the brightness of images displayed on the screen is superior.
Referring to FIG. 1, light emitted from a light source 4 is reflected by a reflection mirror 2 and is projected onto an LCD panel 6 driven by a liquid crystal driving portion 9. Then, an image displayed on LCD panel 6 is magnified onto a screen 10 by a magnifier 8. However, the conventional rear-projection display projects light from a central light source of a high luminance by means of a simple reflection apparatus, i.e. a single mirror. Thus, as shown in FIG. 4A, the brightness at the center of screen 10, which corresponds to the position of the light source, is different than the brightness at other areas of the screen.